HIERARCHIES OF RNA-PROCESSING SIGNALS IN A TRYPANOSOME SURFACE-ANTIGEN MESSENGER-RNA PRECURSOR

Nearly all trypanosome mRNAs are synthesized as polycistronic precursors, from which mature mRNAs are excised by trans splicing and polyadenylation. Polyadenylation of a procyclic acidic repetitive protein (PARP, or procyclin) transcript was studied by transient transfection of constructs bearing a chloramphenicol acetyltransferase gene linked to the PARP intergenic region. Polyadenylation usually occurred at A residues, about 100 bases upstream of a trans-splicing acceptor signal. The wild-type polyadenylation site has a cryptic trans-splicing signal about 100 bp downstream: deletion or inversion of this signal results in polyadenylation at multiple sites, upstream of other cryptic trans-splicing signals. The PARP mRNA precursor appears to contain a hierarchy of possible processing signals, the function of cryptic ones being revealed only when the dominant ones are deleted or moved. Correct polyadenylation can be restored by addition of trans-splicing signals from other loci. The results indicate that polyadenylation is coupled to downstream trans splicing but that the products of the trans-splicing reaction are not necessarily functional mRNAs.